Prostate cancer (PCa) is the most common cancer and the third most common cause of cancer related mortality in men in the United States (Jemal et al., 2006). Androgen ablation remains the standard effective therapy for patients with advanced PCa, inhibiting proliferation and inducing apoptosis in tumor cells (Kyprianou et al., 1990). Unfortunately, after short-term remissions, surviving tumor cells recur with castrate resistant prostate cancer (CRPC) and death usually within 3 years in most men (Gleave et al., 1999). CRPC progression results from mechanisms attributed to re-activation of androgen receptor axis (Knudsen et al., 2009), alternative mitogenic growth factor pathways (Miyake et al., 2000; Culig et al., 2004), and stress-induced prosurvival gene (Gleave et al., 1999; Miyake et al., 1999) and cytoprotective chaperone networks (Rocchi et al., 2004; Miyake et al., 2000). To significantly improve survival in men with PCa, new therapeutic strategies to inhibit the appearance of this phenotype must be developed. It has been observed that numerous proteins are expressed in increased amounts by prostate tumor cells following androgen withdrawal. At least some of these proteins are assumed to be associated with the observed apoptotic cell death which is observed upon androgen withdrawal. (Raffo et al., 1995; Krajewska et al., 1996; McDonnell et al., 1992). The functions of many of the proteins, however, is not completely understood. Clusterin (also known as sulfated glycoprotein-2 (SGP-2) or TRPM-2) is within this latter category.
Clusterin
Clusterin is a cytoprotective chaperone protein that promotes cell survival and confers broad-spectrum resistance to cancer treatments (Chi et al. 2005). In Sensibar et al., Cancer Research 55: 2431-2437, 1995, the authors reported on LNCaP cells transfected with a gene encoding clusterin, and watched to see if expression of this protein altered the effects of tumor necrosis factor α (TNFα), to which LNCaP cells are very sensitive. Treatment of the transfected LNCaP cells with TNFα was shown to result in a transient increase in clusterin levels for a period of a few hours, but these levels had dissipated by the time DNA fragmentation preceding cell death was observed.
As described in U.S. Pat. No. 7,534,773, the contents of which are incorporated by reference, enhancement of castration-induced tumor cell death and delay of the progression of androgen-sensitive cancer cells to androgen-independence may be achieved by inhibiting the expression of clusterin by the cells.
Custirsen
Custirsen is a second-generation antisense oligonucleotide that inhibits clusterin expression. Custirsen is designed specifically to bind to a portion of clusterin mRNA, resulting in the inhibition of the production of clusterin protein. The structure of custirsen is available, for example, in U.S. Pat. No. 6,900,187, the contents of which are incorporated herein by reference. A broad range of studies have shown that custirsen potently regulates the expression of clusterin, facilitates apoptosis, and sensitizes cancerous human prostate, breast, ovarian, lung, renal, bladder, and melanoma cells to chemotherapy (Miyake et al. 2005), see also, U.S. Patent Application Publication No. 2008/0119425 A1. In a clinical trial for androgen-dependent prostate cancer, the drugs flutamide and buserelin were used together in combination with custirsen, increasing prostate cancer cell apoptosis (Chi et al. 2004; Chi et al., 2005).
Hsp90
Heat shock protein 90 (Hsp90) is an ATPase-dependent molecular chaperone required for protein folding, maturation and conformational stabilization of many “client” proteins (Young et al., 2000; Kamal et al., 2003). Hsp90 interacts with several proteins involved in CRPC, including growth factor receptors, cell cycle regulators and signaling kinases like Akt, androgen receptor (AR) or Raf-1, (Whitesell et al., 2005; Takayama et al., 2003). Tumor cells express higher Hsp90 levels compared with benign cells (Kamal et al., 2003; Chiosis et al., 2003), and Hsp90 inhibition has emerged as an exciting target in CRPC and other cancers. Many Hsp90 inhibitors were developed targeting its ATP-binding pocket, including natural compounds such as geldanamycin and its analogs, or synthetic compounds. These agents have been shown to inhibit Hsp90 function and induce apoptosis in preclinical studies of colon, breast, PCa and other cancers (Kamal et al., 2003; Solit et al., 2003; Solit et al., 2002).
Combination Therapy
The administration of two drugs to treat a given condition, such as prostate cancer, raises a number of potential problems. In vivo interactions between two drugs are complex. The effects of any single drug are related to its absorption, distribution, and elimination. When two drugs are introduced into the body, each drug can affect the absorption, distribution, and elimination of the other and hence, alter the effects of the other. For instance, one drug may inhibit, activate or induce the production of enzymes involved in a metabolic route of elimination of the other drug (Guidance for Industry. In vivo drug metabolism/drug interaction studies—study design, data analysis, and recommendations for dosing and labeling). Thus, when two drugs are administered to treat the same condition, it is unpredictable whether each will complement, have no effect on, or interfere with, the therapeutic activity of the other in a human subject.
Not only may the interaction between two drugs affect the intended therapeutic activity of each drug, but the interaction may increase the levels of toxic metabolites (Guidance for Industry. In vivo drug metabolism/drug interaction studies—study design, data analysis, and recommendations for dosing and labeling). The interaction may also heighten or lessen the side effects of each drug. Hence, upon administration of two drugs to treat a disease, it is unpredictable what change will occur in the profile of each drug.
Additionally, it is difficult to accurately predict when the effects of the interaction between the two drugs will become manifest. For example, metabolic interactions between drugs may become apparent upon the initial administration of the second drug, after the two have reached a steady-state concentration or upon discontinuation of one of the drugs (Guidance for Industry. In vivo drug metabolism/drug interaction studies—study design, data analysis, and recommendations for dosing and labeling).
Thus, the success of one drug or each drug alone in an in vitro model, an animal model, or in humans, may not correlate into efficacy when both drugs are administered to humans.